1. Field of the Invention
The present invention relates to a light emitting device using an element in which a light emitting material is sandwiched between electrodes (hereinafter referred to as a light emitting element) and a manufacturing method thereof.
2. Description of the Related Art
In recent years, development of a light emitting device using a light emitting element has been progressed. In the case of the light emitting device, since the light emitting element itself has light emitting ability, a back light such as that used in a liquid crystal display is unnecessary. Thus, thin size and light weight are possible.
There are two types of light emitting devices, that is, a passive type (simple matrix type) and an active type (active matrix), and both the types are actively developed. In particular, an active type light emitting device is noted at present. Also, as a material for a light emitting layer of an organic layer of the light emitting element, there are an organic material and an inorganic material. Further, the organic material is divided into a low molecular system (monomer system) organic material and a high molecular system (polymer system) organic material. Both the materials are actively studied, and film formation using the low molecular system organic material is performed by mainly a vacuum evaporation method and film formation using the high molecular system organic material is performed by mainly a spin coat method.
The organic material is characterized in that its light emitting efficiency is high and low voltage drive is possible, as compared with the inorganic material. Also, since the organic material is an organic compound, various new substances can he designed and produced. Thus, there is a possibility that an element which emits light at higher efficiency is found in accordance with progress of material design in the future.
FIG. 16 is a cross sectional view of a conventional light emitting device. The light emitting device has a construction which bonds a substrate 1601 on which a light emitting element is formed and a sealing substrate 1600 through a seal member 1605. Further, the light emitting element is composed of an anode 1602, an organic layer 1603, and a cathode 1604 and formed so as to sandwich the organic layer 1603 by the anode 1602 and the cathode 1604. Either the anode or the cathode is formed on the substrate. However, the anode is generally formed on the substrate because of the ease in manufacturing. According to the light emitting element, an electron injected from the cathode and a hole injected from the anode are recombined in the center of light emission of the organic film to produce an exciton and the exciton releases energy to emit light when it is returned to a ground state. The light emitting element is located in an enclosed space surrounded by the substrate 1601, the sealing substrate 1600, and the seal member 1605. In this specification, a region surrounded by the substrate, the sealing substrate, the seal member, and the light emitting element is called an enclosed space. Since the light emitting element deteriorates by moisture or oxygen, the enclosed space is filled with an inert gas (nitrogen molecule or noble gas) 1606. There may also be the case where the enclosed space is filled with an organic resin. Reference numeral 1608 denotes a switching TFT (thin film transistor), 1609 denotes a current control TFT, and 1610, 1611, and 1615 denote insulating films. A region of a pixel portion 1620 is shown by arrows in FIG. 16. In this specification, the term sealing substrate indicates a substrate bonded to the substrate through the seal member in order to protect the light emitting element which easily deteriorates by moisture.
Since a material having a high light reflecting property is used for the cathode of the light emitting device, light (incident light 1621) entered from the outside of the light emitting device is reflected from the cathode to produce reflecting light 1622. Thus, there is a case where the face of an observer 1616 is reflected in the cathode such as in a mirror and the observer 1616 recognizes copy-in. In this specification, the term copy-in indicates a state in which the face of an observer, a ceiling, or the like is reflected in a display unit (not shown) of the light emitting device by reflecting light from the cathode and the like. Thus, a circular deflection film 1612 and a polarization plate 1613 are used such that light which is incident from the outside of the light emitting device and reflected from the cathode is not emitted again to the outside. The circular deflection film and the polarization plate are located such that an angle formed by the polarization axes thereof becomes 45°. When such an installation is made, light which was incident from the outside and passed through the polarization plate becomes linearly polarized light. The linearly polarized light is twisted at 45° by the circular deflection film to become elliptically polarized light. The elliptically polarized light is reflected from the cathode and becomes linearly polarized light by the circular deflection film. Since an angle formed by this linearly polarized light and the polarization axis of the polarization plate becomes 90°, reflecting light is absorbed in the polarization plate. Thus, the circular deflection film 1612 and the polarization plate 1613 are provided in the light emitting device such that the copy-in is invisible to an observer 1616.
Therefore, when the circular deflection film 1612 and the polarization plate 1613 are used in the light emitting device as shown in FIG. 16, light is absorbed in the polarization plate 1613 at about half the amount (38% to 48%) thereof. Further, light emitted from the organic layer 1603 is also absorbed in the polarization plate at about half the amount thereof. Thus, there is a problem in that the brightness recognized by the observer 1616 is decreased at about half.